The NH-D15 is going to block off the top PCI-E slot on X99 motherboards.
If you want to avoid that you need to send the NH-D15 back and get the NH-D15S instead.

As you can see in this video the S is offset to one side so it does not block PCI-E ports.

I have the NH-D15S myself on an older Rampage IV board and there is not more then 2-3 cm between the cooler and the top video card.
With the fanclips on there is about 1 cm of room if there is even that.

nah he doesn't need to send back NH-D15, as it works fine with X99 motherboards. yes it's a bit tight fit, but even if GPU has a backplate, like mine does, there's still about 3-5mm gap between GPU and D15 so it's fine. i already had 3 different GPUs on X99 with NH-D15 and they all fit fine Edited by Glottis - 6/2/16 at 2:23am

nah he doesn't need to send back NH-D15, as it works fine with X99 motherboards. yes it's a bit tight fit, but even if GPU has a backplate, like mine does, there's still about 3-5mm gap between GPU and D15 so it's fine. i already had 3 different GPUs on X99 with NH-D15 and they all fit fine

It depends on which X99 board we are talking about as some has more space then others.
Take the Rampage boards if you use an NH-D15 on those you WILL lose the top PCI-E slot.

The Strix has more space so the NH-D15 might be able to clear the top slot.
Again i would recommend you get the S version just to avoid that problem all together.Edited by shilka - 6/2/16 at 2:46am